^ Some monitors require 120, or 240 VAC as an input, But some require a AC to DC converter (brick)

If your monitor Requires a "Brick" Power pack: short answer NO you can not use a power pack rated @ 12V/500 mAmps inplace of one rated at 12V/4Amps. It will not power your monitor and the DC output would be very low - You could damage your monitor. Also the brick will Overheat if left attached for very long.